1. Field of the Invention
This invention relates to the art of calcining finely divided naturally-occurring hydrated kaolin clay to produce a calcined clay pigment having a higher brightness than the starting clay and useful, for example, as a filler for paper or as a pigment for paper, plastics or paint.
Calcined kaolin clay is widely used by the paper and paint industries as a filler or pigment. Desirable properties of a calcined clay pigment or filler are high brightness and low abrasiveness. The principal demand is for calcined clay pigments having a brightness (as measured by the well-known TAPPI method) of at least 90 percent, preferably above, and an abrasiveness (as determined by the Valley method) below 100, preferably below 50.
The calcination treatment, which may be carried out in a rotary calciner with countercurrent flow of hot air or in a stationary furnace such as a muffle or Nichols Herreshoff furnace, is usually applied to a finely pulverized refined clay. During calcination, the kaolin undergoes a well-defined endothermic reaction associated with loss of water of crystallization when the clay temperature reaches about 1350.degree.F. This results in an amorphous essentially anhydrous material usually referred to as "metakaolin." If the clay temperature is further increased, the metakaolin undergoes a characteristic exothermic reaction at about 1800.degree.F. At temperatures above about 2200.degree.F. new crystalline phases begin to form.
It is well known that the extent to which kaolin clay is brightened by calcination depends upon the calcination temperature. Thus, when a kaolin clay is calcined at temperatures in the range of about 1800.degree.F. to 2100.degree.F. the resulting product will be brighter than it would be if the same clay had been calcined at temperatures in the range of 1350.degree.F. to 1650.degree.F. for about the same period of time. However, all factors being constant, clay calcined at 1350.degree.F. to 1650.degree.F. will be less abrasive than the clay would be if calcined at a temperature in the range of about 1800.degree.F. to 2100.degree.F. At temperatures above about 2200.degree.F. or 2300.degree.F. the clay will be excessively abrasive unless the heating time is limited. Heating of kaolin clay at temperatures in the range of about 300.degree.F. to 1100.degree.F. usually darkens the clay. To the best of my knowledge, it is not fully understood why variations in calcination temperature have these effects on clay brightness.
The commercial production of very high brightness calcined clay pigments, e.g., pigments having brightness above 90 percent, usually involves calcining kaolin clay at temperatures of the order of 2000.degree.F. Less bright and less abrasive pigments are produced at lower calcination temperatures, e.g., 1500.degree.F.
Calcination adds appreciably to the cost of the clay. In view of the current energy shortage it would be desirable to produce very high brightness pigments at lower temperatures than those currently used. This would also provide the added benefit of reducing the abrasiveness obtainded at any given calcination temperature. thus, it would be desirable to calcine a clay at a temperature of the order of 1600.degree.F. to 1700.degree.F. and produce a pigment that would be at least as bright (but less abrasive) than the pigment would be if it were calcined at a temperature of about 2000.degree.F. with a greater consumption of fuel. Conversely, using temperatures in the range of about 1800.degree.F. to 2100.degree.F., it would be desirable to produce brighter pigments than those obtained at similar elevated temperatures utilizing presently known calcination technology.
2. Prior Art
The following patents describe prior art techniques for preparing calcined clay pigments and the like from naturally-occurring kaolin clay.
U.S. Pat. No. 2,256,528 to Rowe
U.S. Pat. No. 3,309,214 to Podschus et al
U.S. Pat. No. 3,014,836 to Proctor
U.S. Pat. No. 3,021,195 to Podschus et al
U.S. Pat. No. 3,383,438 to Allegrini et al
Following are patents dealing with the recovery of clay from paper mill sludges by processing which includes one or more calcination steps.
U.S. Pat. No. 3,188,751 to Sutton
U.S. Pat. No. 3,765,921 to Puskar
It is well known that clay may be brightened by volatilizing from impurities as the chlorides at elevated temperature, optionally adding carbon to the clay before introducing chlorine gas or a source of chlorine. In this case, the carbon is added to facilitate the formation of a volatile iron chloride at a lower temperature.